1. Field of the Invention
The present invention relates to chemical retardants which may be applied by various application methods including aerial bombardment of forest, rangelands, brushland and other areas for preventing, extinguishing or suppressing fires therein.
2. Description of the Prior Art
Forest, brush and rangeland fires causes enormous damage each year. Not only is the direct property loss due to such fires enormous, but associated soil erosion and watershed problems are also significant. It is important, therefore, to minimize and control the spread of forest fires whenever possible.
Forest fire retardants may be classified as short-term or long-term retardants. Short-term retardants, as defined in U.S. Pat. No. 3,553,128, rely solely upon the water they contain to retard combustion. Long-term retardants contain, in addition to water, a chemical that effectively retards flaming combustion even after the water has evaporated.
At present, the most commonly used retardant chemicals are ammonium salts such as monoammonium orthophosphate, diammonium orthophosphate, condensed ammonium phosphate moieties which exist in fertilizer solution, ammonium sulfate and the like. SUch ammonium salts are usually employed in aqueous solution to facilitate spraying from mobile ground application equipment or for aerial bombardment from fixed-wing aircraft or helicopters. Functionally, the water serves primarily as a carrier for the chemical retardants.
It is common practice to thicken long-term retardants with additives in order to improve their capabilities with respect to placement or confinement on the target, the target being the timber or other foliage which constitutes the fuel for combustion. Such additives also can improve adherence of the retardants to the fuel surface; can help retain moisture; can create a moisture barrier buildup between the fuel and the flame; and can improve deposition of the retardant on the fuel surface.
In the aerial application of liquid materials for preventing, retarding and suppressing fires, such as commonly undertaken using fixed-wing aircraft, the prior art has recognized that solutions of low viscosity, when dropped from substantial elevations, usually tend to atomize and therefore cannot be relied upon to descend with sufficient concentration on specific target areas. It has been found that liquid fire suppressing materials having high viscosity, for example 1,500 to 2,500 centipoises, and cohesive properties, when dropped from substantial elevations, tend to hold together such that these liquids can be confined to a specific target area and will descend thereupon with sufficient concentrations to be effective.
As pointed out in U.S. Pat. No. 3,196,108, most thickening agents for increasing the viscosity and cohesive properties of aqueous solutions are not compatible with the most effective fire suppressing chemicals available. Furthermore, it is stated therein that many of the fire retardant chemicals which are compatible with some thickeners have such a low fire retarding capacity that great amounts of these fire retardant materials must be employed when used with compatible thickeners. It has also been found, according to U.S. Pat. No. 3,196,108, that some of the fire suppressing materials used are effective when wet but have little or no value when dried.
Among the additives known for use in modifying the rheological properties of ammonium phosphate solutions are the galactomannan gums, sometimes called polygalactomannans. U.S. Pat. No. 3,634,234 describes the use of galactomannan gums in such solutions. The galactomannan materials are polysaccharides, generally termed hemicelluloses, and are long-chain polymers of galactose and mannose units. They are gum-like materials generally found in plant seeds. Examples of the galactomannans are guar gum; locust bean gum; and tara gum. These materials typically comprise about 5-15 percent by dry weight of the ammonium phosphate composition according to U.S. Pat. No. 3,634,234.
For reasons not entirely understood, certain galactomannan materials have been found to be incompatible with ammonium sulfate (an effective fire retardant chemical) when employed in forest fire retarding solutions. For this reason, it has been traditional to thicken ammonium sulfate solutions with clay instead of polygalactomannans. U.S. Pat. No. 3,196,108 exemplifies typical prior art ammonium sulfate solutions containing attapulgite clay thickeners.
In air drop test comparisons, gum-thickened retardants have been found to exhibit improved rheological properties over clay-thickened retardants. For example, gum-thickened retardants were found to produce larger droplet sizes and to experience less erosion and drift during a drop thus resulting in more concentrated patterns. Also, drop times were shorter for gum-thickened retardants. With these improved properties, higher effective drop heights may be achieved. This increases safety in air dropping operations and could enable larger aircraft to be used for air dropping. However, due to the aforementioned incompatibility problem with ammonium sulfate, the advantages accuring from the use of polygalactomannan gum thickeners have heretofore inured only to the benefit of ammonium phosphate solutions.
If it were possible to gum-thicken aqueous ammonium sulfate solutions, it is believed that a superior product (relative to clay-thickened ammonium sulfate solutions) could be produced. Furthermore, since ammonium sulfate solutions have been found inferior to ammonium phosphate in prevention of glowing combustion, a further improvement might be realized by admixing ammonium sulfate and ammonium phosphate in a single fire retardant composition. Admixtures of ammonium sulfate and diammonium phosphate in aqueous fire retardant compositions are disclosed in U.S. Pat. No. 3,409,550. The preferred water-miscible thickener in that disclosure was carboxymethyl cellulose. Polygalactomannans were not disclosed.
It is an object of the present invention, therefore, to provide aqueous ammonium sulfate solutions containing certain derivatives of polygalactomannans which are compatible with ammonium sulfate and which afford the necessary rheological properties of the resulting fire retardant solution. Another object of the present invention is to provide forest fire retardants comprising aqueous solutions of such polygalactomannan derivatives in combination with admixtures of ammonium sulfate and ammonium phosphate. These and other objects of the present invention will become apparent from the following description.